Interesting movie, but ending ? (spoilers)

I did not really understand the ending.
I gather that she shot the man instead of the girl.
But the question of why I did not understand.
Other than the ending the mood and feel of the movie
was very well done. It seemed like it went on for days,
and I don't mean anything bad about that, but it has a
suspension of time feel to it. I would rate it about a 4-5/10
kind of average.